1. Vertical Axis Wind Turbine Noise
Charlie Pearson
Dr. Will Graham
Dr. Tamás Bertényi
Charlie Pearson
1st year PhD

2. What is a VAWT?
Drag Type
Lift Type

3. Why VAWTs?
Ideal for built environment…

4. Built Environment Application
Issues:
Turbines are in close proximity to people.
Noise is a potentially significant problem
Objectives:
Identify dominant mechanisms of noise generation.
Investigate possible methods for reducing radiated sound.
Turbine on your house or your office  noise will be a problem
What is the dominant source of noise on a VAWT
At the moment the noise mechanisms are very poorly understood
No detailed experimental measurements have been made.
How might we reduce that noise?

5. How does a lift-type VAWT work?
VBlade
VRel
FLift
FTang. V∞
VBlade
VRel
FLift
FTang.
Angle of attack, α V∞
VBlade
VRel
FLift
FTang.
In order to understand the noise generation we need to first understand how a VAWT produces torque… 1
Angle of attack generated by combination of wind speed and blade speed
Produces a tangential force and therefore torque 2
Varies as a function of blade position 3
The magnitude of the variation is dependent on the TSR 4 5
The variation is greater at lower TSR
Even at higher TSR the blades are likely to go into stall
Key conclusion is that the blade loading is fundamentally unsteady.
Key Parameter: Tip Speed Ratio
TSR= Blade speed / Wind Speed

6. Where does noise come from?
Kinetic energy is converted into acoustic energy by three mechanisms:
By forcing the mass in a fixed region of space to fluctuate, e.g. a loudspeaker in a large baffle.
By forcing the momentum in a fixed region of space to fluctuate, e.g. a solid object vibrating after being struck.
By forcing the rates of momentum flux across fixed surfaces to vary, e.g. turbulent jet.
In order to understand why unsteady loads are important we need to look at how noise is generated…
Forcing the mass to fluctuate: Loudspeaker in baffle, pulsating sphere
Momentum fluctuation: Vibrating solid object, oscillating sphere
Momentum flux fluctuation: turbulent fluctuations
These are in order of strength (high to low) for low Mach number flows.
VAWTs operate at low Mach
Definitions from MJ Lighthill, 1952, On Sound Generated Aerodynamically, Proc. R. Soc. Lond. A, Vol.267,
Jet noise image from:

7. Where does VAWT noise come from?
Dipole noise due to fluctuating blade loads dominates at low Mach number.
Need to know the blade loads in order to determine the noise analytically.
This presents some significant problems…
Now the situation is becoming clearer…
Unsteady loads drive the sound field
If we know the blade loads then we can determine the sound field
How do we go about calculating the sound field?
It is pretty tricky

8. CFD Modeling? Flow is fundamentally unsteady Flow is 3D
Flow is turbulent and has a wide range of length and time scales
Flow is VERY complicated!
Difficult to determine the most significant noise sources.
First attempt might be to try CFD…
As we have seen the flow is:
Unsteady
Effect of blade tips make the flow very 3D in nature
To resolve all acoustic wavelengths we need to have high resolution on the blades – TIME AND SPACE
Flow is complex!
Unless we can perform very high fidelity simulations we are not likely to get an accurate picture of the sound field.
Image from Scheurich, Fletcher and Brown, (2010), The influence of Blade Curvature and Helical Blade Twist on the Performance of a Vertical Axis Wind Turbine, 48th AIAA Aerospace Sciences Meeting, Orlando, USA

9. An alternative approach...
Use analytic acoustic model for moving point source.
Guess the blade loads
Sinusoidal variation with azimuth?
Blade/wake interactions?
Dynamic Stall?
Predict the resulting sound field
Far field:
Near field:
MV Lowson, (1965), The Sound Field for Singularities in Motion, Proc. R. Soc. A, Vol.286,
Use moving singularity model developed by Lowson.
Keeps the key components while avoiding unnecessary complication
Key component is the fluctuating force component in the far field term
Set up the rotor geometry and the observer location
Guess the blade loads
4. Identify key characteristics of sound field for comparison with experimental data.

10. Loading Profiles Tested
Define normal and tangential blade loads as a function of azimuth:
Constant Load
Sine Wave
Blade Vortex Interaction
Zero wind load
Constant drag load
Loads you would get if there were no blade/wake effects.
Load due to the blade passing close to a coherent vortex.
Constant load:
Good for calibration
Sinusoidal load:
Sort of loading you would get if there were no wake interactions
Good representation of the upstream half of the rotor
BVI load:
Loading profile is dependent on the strength of the vortex and the blade/vortex separation
Strongly directional sound field

11. Experiments
Use acoustic array in Markham wind tunnel to locate dominant noise sources.
Investigate parameters for noise reduction
Blade tip shape
Spoke geometry
Variable radius
Blade sweep / helical blades
Measure the sound field using the acoustic array in the Markham tunnel.
Based on results of initial measurements, investigate some design parameters to see if the noise can be reduced.
Blade tips: change structure of the rotor wake
Spoke geometry: cylindrical / aerofoil
Radius: Impact on tip vortices
Helix: Twist spreads load around rotor, smoother loading?

12. 3 Straight blades
Motor driven so we can control the speed more accurately
Measure the unsteady torque on the shaft.
Measure the sound field using the acoustic array

13. Array Measurements
Sijtsma, P (2004), Experimental techniques for identification and characterisation of noise sources, National Aerospace Laboratory NLR Report, NLR-TP
Use phase relationship between signal received at multiple microphones.
Acoustic array consists of multiple microphones which simultaneously record the sound from an object.
By comparing the signals at each microphone it is possible to estimate the source location.
We want to attempt to do for VAWTs what Peter Sijtsma has done for HAWTs.
Sijtsma, P, Oerlermans, S and Holthusen, H (2001), Location of Rotating Sources by Phased Array Measurements, NLR-TP

14. Plan of action
Measure sound field in the wind tunnel using acoustic array.
Compare results with computer model predictions.
Investigate dominant noise sources in detail.
Inform the design of quieter VAWTs
Questions?